Conventionally, there has been recommended by the International organization for Standardization/International Electrotechnical Commission (ISO/IEC), for example, ISO/IEC 14496-2(MPEG-4 Visual) as a technology which codes the moving-picture signals. The MPEG-4 Visual is a multimedia coding system and a technology which may code various kinds of video materials.
The image coding technology has been realized by three technologies, namely, “Motion compensation prediction coding system”, “Discrete cosine transform”, and, “Variable-length coding”.
Hereinafter, these technologies will be described.
(1) Motion Compensation Prediction Coding System
First, the input image picture is compared with the previously encoded picture so as to predict the motion between those pictures , and an input image picture is predicted from the motion quantity and the previously encoded picture. Then, the difference (predicted error signal) between the predicted image and the input image picture is calculated, and the predicted error signal and the motion quantity which has been previously obtained is sent to the receiving side. Image information may be sent using a small amount of data through predicting the motion in the above-described manner. Here, Apart from this motion compensation prediction coding system, there is an intra coding system which does not code the difference between an input image picture and the previously encoded picture, but the image data themselves. The intra coding system has been often used when the arithmetic errors are initialized, or when degradation in the picture quality caused by transmission errors is recovered, and since the predicted errors are not used, the amount of generated codes is increased in this system.
(2) Discrete Cosine Transform (DCT)
The discrete cosine transform converts the above-described predicted error signal into the frequency domain. Image information may be transmitted by a further smaller amount of data through combination of the following feature and the variable-length coding system, as the feature of this system is that the power concentrates on a specific frequency domain (low frequency domain) when the predicted error signal is converted into the frequency domain.
(3) Variable-Length Coding System
The variable-length coding system is a system intended to reduce an average code length by expressing the events with high frequency of occurrence in terms of short code lengths, and of events with low frequency of occurrence in terms of long code lengths, making use of bias the events when there is bias in the frequency of occurrence with regard to data to be coded. Image information may be transmitted by a small amount of data, using the above-described system.
The above-described three element technologies are not applied to the whole image picture, but to each unit divided into coded blocks (macroblocks) of 16×16 pixels. Regarding to the above-described motion, switching between the motion compensation prediction coding system and the intra coding system is performed in macroblocks.
Recently, radio transmission technologies for moving-picture signals have been attracting much interest along with wide spread of radio communication terminals such as cellular telephones. Coding technologies with transmission error tolerance have been indispensable for radio transmission, as the radio transmission has a higher transmission-error rate compared with that of wire transmission.
Among technologies which control image degradation caused by transmission errors, there is a technology which is called a videopacket. The technology suggests transmission of coded data of a plurality of macroblocks as one transmission unit (video packet). The videopacket comprises of a video packet header information and coded data of an arbitrary number of macroblocks. The video packet header includes a position address (the address denotes at which position the pertinent macroblock is in the picture) of a first macroblock, and a parameter necessary for decoding.
As an example of the methods for forming the videopacket is the Japanese Patent Published Application No. Hei-7 (1995) -014514 in which the videopacket is formed when the amount of generated codes in macroblocks are accumulated and reaches a predetermined amount of codes. According to this method, in the video packet which has no motion in the picture such as backgrounds, there is a small amount of codes comprises many macroblocks, and in the video packet which has large motion in the picture, there is a large amount of codes comprises few macroblocks. Therefore, a degraded range may be minimized as parts with large motion comprise a smaller amount of macroblocks even when there are occurred errors in the video packet caused by transmission errors. That is, the deterioration is not noticeable as there is no motion, though the background parts include a lot of macroblocks. Bits may be efficiently assigned by applying the above method.
Generally, macroblocks with large amount of codes has large change in case of the motion compensation prediction coding system. That is, the macroblocks may be considered to represent parts with large motion. When there are generated transmission errors in the macroblocks, it is considered that effects on the image quality of regenerated images are large. That is, macroblocks with a large amount of generated codes is considered to have higher significance. On the other hand, degradation in the picture quality caused by transmission errors may be recovered, though there are a larger amount of generated codes because of no use of the prediction in the case of the intra coding. Then, this is also considered to have higher significance.
As another technology to improve the error tolerance, there is a technology (the Japanese Patent Published Application No. Hei-9 (1997) -214721) in which retransmission is performed according to the significance of images. In this technology, still images are converted into the frequency domain; variable-length coding of components with higher frequency than a predetermined frequency and that of components with lower one are independently performed, and two kinds of transmission frames are formed. Retransmission request is controlled only for the components with lower frequency rather than for the components with higher frequency when these frames are transmitted and transmission errors are found at the receiving side. Thereby, transmission efficiency may be improved, though there are acceptable degree of some degradations in the image quality.
However, there have been the following problems in the conventional image transmission systems.
Generally, when hierarchical coding is performed according to the frequency domain and the resolutions of images, overhead information is required by division of information and hence, a division loss caused by the requirement will be generated. That is, it is required to double the header information of each transmission unit when dividing into two, for instance, because the coded data are usually composed of header information and macroblock coded data at each transmission unit. This amount becomes an overhead.
Moreover, hardware for decoding processing of each hierarchy, or software resources for hierarchical coding is required, resulting in complex configuration of the apparatuses.
Recently, technologies for radio transmission of moving pictures have been studied, and a ratio of the above-described division loss has become so large, so that it may not be neglected, as the transmission rate of radio transmission paths is smaller than that of wire nets. Moreover, it is required for mobile terminals, which may receive moving pictures, to control the consumption low in order to make communication time longer as much as possible. However, the communication time of the terminals will be effected by the increase of hardware and software resources on account of hierarchical coding.
The object of the present invention is to provide an image coding apparatus and system in which hierarchical coding is not required; the significance of moving-picture coded data may be decided with an extremely simple configuration; and retransmission may be effectively controlled.